The U.S. National Transportation Safety Board has received word that Egypt accepted its offer to assist in the Metrojet crash investigation, a U.S. source with knowledge of the investigation said.
The NTSB is waiting for more specific information on when and where the plane's engines will be brought to be examined before it dispatches a team, the source said.
In an interview with CNN's "The Situation Room" Tuesday, Egyptian Foreign Minister Sameh Shoukry said his country had approved an application for American investigators to participate.
"Egypt has already accepted the application of American investigators that are associated with the manufacturers of the engine to become part of the investigative team, and they are free to incorporate any advisers they deem as necessary for them to undertake the responsibility," Shoukry said.
The plane's engines were built by American manufacturer Pratt & Whitney.
Getting access to the wreckage could mark a significant development in the investigation, allowing U.S. officials -- who've so far relied on intelligence to assess what happened to the plane -- to see the physical evidence needed to provide definitive proof.
It's unclear whether other U.S. investigators would join the NTSB team.
Asked whether FBI agents could be involved, Shoukry said international investigation regulations allow Americans to tap advisers for their team.
"The FBI has offered forensic assistance and other services to our partners in Egypt and Russia, and stands ready to assist," the agency said in a statement.
Communications intercepts provide clues
U.S., British and other authorities believe that the Russian Metrojet that crashed in Egypt was likely downed by a bomb planted on the plane, even though Russian and Egyptian investigators haven't shared any of the physical evidence up to now.
U.S. officials have been trying to connect the dots with the data points they have so that they can learn from what happened and better safeguard American flights.
"Given the ongoing investigation, we are particularly focused on what happened, understanding what happened, and what more we could do in that region," Secretary of Homeland Security Jeh Johnson said on Saturday.
American government analysts have been using communication intercepts and data to build a plausible explanation of how a potential plot could have transpired, multiple U.S. officials told CNN.
But the officials, who shared details of U.S. national security discussions regarding the downed plane, stressed that they are no closer to a true understanding of what occurred.
They expressed great caution about making an assessment given the lack of evidence, noting that terrorists were initially thought to have brought down TWA Flight 800 when a technical error was later shown to be responsible.
The preliminary view comes from analysts looking at, among other sources, the infrared reading of a heat flash captured by a U.S. military satellite while the plane was in flight, video and photos from the crash site and expertise on the effect of various explosives.
In addition, the U.S. is examining messages from communication intercepts after the attack from ISIS in Sinai to ISIS leadership back in Syria detailing elements of the incident.
British Foreign Minister Philip Hammond echoed that view on Tuesday when he told reporters in Washington that the U.K. is not aware of any explosive device having been found by Russian or Egyptian investigators, though he acknowledged that the U.K. has no access to the progress of the investigation.
For now, the U.S. does not see evidence that a plot was ordered by ISIS leadership in Raqqa but rather was most likely planned and carried out by militants who operate in Sinai. So far "there is no indication" that ISIS leadership ordered the attack, multiple official said.
U.S. officials believe social media accounts connected to ISIS in Syria and Iraq have been relatively quiet for several days now because they know they may be targeted for retribution. Officials said it is also possible that the perpetrators could be local militants who have self-radicalized and affiliated themselves with ISIS.
But all the officials emphasized that without direct access to the technical data, the plane's wreckage and the bodies, the U.S. is not reaching any final conclusions.
How could bomb have been planted?
For now, the most likely scenario is that jihadists planted a bomb with a timer on the plane by someone who had access rather than a passenger sneaking it through the security system, three U.S. officials familiar with the latest intelligence analysis told CNN. The timer would have been set for enough time to initiate the explosion only after the plane had taken off, according to the officials.
Hammond noted that "you don't need" a sophisticated capability and that a small bomb with a straightforward timer is sufficient to bring down an aircraft, though he said he was not speaking directly to what the British government thinks the potential device was.
"You only need a couple of pounds of explosives and a timer and you are there. We are not talking about rocket science, we are talking about schoolboy physics," Hammond observed.
A bomb of this type would likely be made in part by an easily attainable military-grade explosive like C4, according to two of the American officials. The feeling, U.S. officials explained, is that only explosives of that strength could have caused the disaster.
Explosives like C4 have been used in other attacks by ISIS in Sinai, according to Zack Gold, an expert on ISIS in Sinai. Gold said just this year that the Egyptian military intercepted shipments of explosive materials that included C4.
Mohannad Sabry, an expert on the region, says there are thousands of kilos of explosives available in the Sinai. Sabry said militants in Sinai have also proven capable of making their own explosive material.
U.S. analysts are looking at what is known about the strength of the heat flash registered by a U.S. satellite at the time of the explosion, as well as what the U.S. believes is the intensity of the sound on the plane's cockpit voice recorder.
The officials stressed, however, that unless the Russians and Egyptians share the information they have, the U.S. may never definitively know what happened.
"The lack of transparency demonstrated by Russia over time, in our minds means we may never be fully informed about the cause," said one of the officials.
Another senior intelligence official cautioned that much of the evidence so far amounts to "hearsay" with no access to debris, bodies or the cockpit voice recorder that normally would be a key part of a crash investigation at this point.